Peat depth as a control on peatland ecohydrological resilience

Shallow peatlands (average peat depth <40 cm) exhibit differences in key structural and hydrophysical characteristics from their deeper counterparts. They generally exhibit higher bulk density, lower organic matter content, lower hydraulic conductivity, greater tree density and height, and lower microtopographic complexity. These differences mediate the strength of ecohydrological feedback mechanisms, generally resulting in weaker mechanisms with a regulatory function (negative feedbacks) and stronger mechanisms that have a destabilizing function (positive feedbacks). Ultimately, these differences in peatland form and function result in systems that have a profound contrast in ecohydrological behaviour, exhibiting more frequent water table fluctuations, longer periods when the water table is both above and below the optimum depth, and shorter periods where the surface soil water tension is in equilibrium with the water table. We hypothesize that this leads to greater decomposition, lower productivity, and thus a smaller net carbon sequestration. As a consequence, these shallow peatlands are disproportionately more vulnerable to disturbances, such as drought and wildfire. This can perpetuate a cycle of vulnerability, which prevents shallow systems from obtaining the depth associated with greater resilience. By studying these vulnerable systems we can learn what environmental conditions herald a regime shift associated with a loss of resilience and a degrading peat carbon stock.